Light fixtures creating various effects are getting more and more used in the entertainment industry in order to create various light effects and mood lighting in connection with live shows, TV shows or as a part of an architectural installation.
Typically, such variable color light sources comprise a plurality of individually controllable light sources such that each individually controllable light source emits light of a predetermined color. For example, in an RGB system, the variable-color light source may comprise individually controllable light sources of the most common primary colors red, blue, and green. By controlling the relative brightness of the respective individually controllable light sources of the different primary colors almost any color in the visible spectrum may be generated by means of an additive mixing of the respective primary colors, resulting in output light of the desired color and intensity.
U.S. Pat. No. 6,016,038 and U.S. Pat. No. 6,806,659 disclose systems and methods relate to LED systems capable of generating light, such as for illumination or display purposes. The light-emitting LEDs may be controlled by a processor to alter the brightness and/or color of the generated light, e.g., by using pulse-width modulated signals. The disclosed illumination device comprises LEDs including at least two different colors; a switching device, interposed between the LEDs and a common potential reference, including at least two switches corresponding to current paths of the two different color LEDs; a controller that opens and closes the switches according to a predetermined duty cycle; and a hand-held housing with a compartment for containing a power source and the common reference potential, as well as a lens assembly for reflecting light from the LEDs. The LEDs of different colors are provided in LED sets each preferably containing serial/parallel array of LEDs of the same color and these LEDs are individual controllable by the controller.
The illumination devices as disclosed by U.S. Pat. No. 6,016,038 and U.S. Pat. No. 6,806,659 can also be used to provide a white illumination device where the color temperature can be varied for instance as described in U.S. Pat. No. 6,636,003. U.S. Pat. No. 6,636,003 discloses a LED arrangement which produces a color temperature adjustable white light. The LED arrangement includes one or more white LEDs and a first drive circuit operable to supply a first drive current to the one or more white LEDs such that a white light is output at a desired intensity. The LED arrangement further includes one or more colored LEDs arranged such that a light output from the one or more colored LEDs combines with the white light to produce a resultant light having a desired color temperature. The colored LEDs are driven by a second drive circuit which supplies a second drive current to the one or more colored LEDs such that a colored light is output at a desired intensity, the intensity of the colored light output from the one or more colored LEDs being adjustable so as to adjust the color temperature of the resultant light.
Multi-colored illumination devices as disclosed by U.S. Pat. No. 6,016,038 and U.S. Pat. No. 6,806,659 can generate many different colors, however the overall brightness of the satiated colors (like red, green or blue) are reduced as a smaller number of light sources are activated when such device provides a satiated color. In some situations the illumination device is intended to provide only one single color and in order to enhance the overall brightness of the satiated color the illumination device is then alternatively provided with a single array of light sources emitting the same color instead of three arrays of light sources having different color.
However when light from several of such illumination devices are combined into one illumination (e.g. in order to illuminate architectural structure or a large stage area with the same color) color differences might occur, as the light sources used in two different illumination devices might differ. The reason for this is fact that it is difficult to manufacture light sources emitting the exact same color and brightness. This problem is a widely known issue in connection with LEDs and the LED manufacturers have assisted the illumination device providers by pre-sorting or binning the LEDs into smaller ranges of variability prior to shipment. The smaller range of LED input stimuli has assisted the assembler in producing a target output color. Acceptable color rendering is still a demanding task because even the bins have a sizeable range of the performance variations and the cost of pre-sorted binnings are much higher than regular binnings.
It is known that it is possible to compensate for the differences in color and brightness of the same type/color of light sources in two different multi-color illumination device by using the two other types/colors light sources colors to align the overall color and/or brightness of the two illumination devices. The known multi-color illumination device can be adapted to a bright single color illumination device which can compensate for the color/brightness differences by increasing the number of light sources emitting the single color and reducing the number of the other light sources. However this requires redesign of both software and hardware as at least printed circuited boards, drivers circuit, power supplies need to be dramatically redesigned which will increase manufacturing costs.
Further, due to the varying characteristics and potential non-linearity of the individual light sources, it is difficult to obtain a precise color control at different brightness values. This typically requires a cumbersome manual adjustment of the individual sources or a complicated and costly feed-back control of the light sources. For example, it is cumbersome to control the individual potentiometers such that the overall brightness of a variable-color light source assembly is varied while keeping the color (e.g. the hue and saturation) constant. In a multicolored illumination device these effects can be reduced by calibrating the illumination device for instance as described in WO2007/062662, U.S. Pat. No. 7,626,345, WO2001/052901, US 2004/135524 or WO 2009/034060.
WO 2007/062662 discloses a control device for controlling a variable-color light source, the variable-color light source comprising a plurality of individually controllable color light sources. The control device comprises a control unit for generating, responsive to an input signal indicative of a color and a brightness, respective activation signals for each of the individually controllable color light sources. The control unit is configured to generate the activation signals from the input signal and from predetermined calibration data indicative of at least one set of color values for each of the individually controllable light sources.
U.S. Pat. No. 7,626,345 discloses a manufacturing process for storing measured light output internal to an individual LED assembly, and an LED assembly realized by the process. The process utilizes a manufacturing test system to hold an LED light assembly a controlled distance and angle from the spectral output measurement tool. Spectral coordinates, forward voltage, and environmental measurements for the as manufactured assembly are measured for each base color LED. The measurements are recorded to a storage device internal to the LED assembly. Those stored measurements can then be utilized in usage of the LED assembly to provide accurate and precise control of the light output by the LED assembly.
WO2002/052901 discloses a method and luminaire for driving an array of LEDs with at least one LED in each of a plurality of colors in a luminaire. This method controls the light output and color of the LEDs by measuring color coordinates for each LED light source for different temperatures, storing the expressions of the color coordinates as a function of the temperatures, deriving equations for the color coordinates as a function of temperature, calculating the color coordinates and lumen output fractions on-line, and controlling the light output and color of said LEDs based upon the calculated color coordinates and lumen output fractions.
US 2004/135524 relates to a method and system for compensating for color variations due to thermal differences in LED based lighting systems. The method and system involves characterizing the LEDs to determine what PWM (pulse-width modulation) is needed at various operating temperatures to achieve a desired resultant color. The characterization data is then stored in the microprocessor either in the form of a correction factor or as actual data. When an operating temperature that is different from a calibration temperature is detected, the characterization data is used to adjust the PWM of the LEDs to restores the LEDs to the desired resultant color.
WO 2009/034060 relates to a method for the temperature-dependent adjustment of the color or photometric properties of an LED illumination device having LEDs or LED color groups emitting light of different colors or wavelengths, emitting light of the same color or wavelength within a color group, the luminous flux portion thereof determining the light color, color temperature, and/or the color location of the light mixture emitted by the LED illumination device, characterized by measurement of the board temperature and/or junction temperature of at least one LED, determination of at least one temperature-dependent value determining the emission spectra E(?) of the variously colored LEDs as a function of the wavelength of the variously colored LEDs from calibration data stored for each of the variously colored LEDs, determination of the luminous flux portions of the variously colored LEDs for a light mixture comprising a prescribed light color, color temperature, and/or color location at the measured temperature as a function of the at least one temperature-dependent value determined, and adjustment of the determined luminous flux portions at the variously colored LEDs.